Go to Anistar City and find the old man who wants to relax with one of your level 5 or lower Pokemon. Give him any Pokemon that is level 5 or lower. Visit him again after a few days. The only thing in his house will be a Pokeball containing your loaned Pokemon, with a note attached from the man. He will leave a Comet Shard for you, which sells well in Lumiose City.